This issue's special section is the first of a two-part series on recent research conducted at the Aztec city-state of Otumba and the neighboring polities of Tepeapulco to the east and Teotihuacan to the west. Otumba is an extremely important site for the archaeological study of the Late Aztec period because, as Thomas H. Charlton, Deborah L. Nichols, and Cynthia L. Otis Charlton point out in their introduction, it is one of the few places in the Valley of Mexico where late pre-Columbian remains are not obscured by post-Hispanic occupation. Research directed by Charlton at Otumba began in the early 1960s and continued in the late 1980s under the direction of Charlton and Nichols, followed by laboratory and technical analyses of the archaeological materials as well as ecological and ethnohistoric studies. Charlton and his collaborators have amassed a prodigious amount of data, which allows them to address a wide range of issues revolving around the general question of the processes involved in the origins and development of the city-state in central Mexico during the Early and Middle Postclassic periods. They do so within a theoretical context that engages the two most prevalent general models proposed to account for the rise of Postclassic city-states in central Mexico.